Wafer inspection systems are well known in the art. One conventional system, embodied in U.S. Pat. No. 5,699,447 uses normal illumination and bright-field detection (i.e., the illumination approaches the wafer at ninety degrees thereto). Another type of conventional system, as embodied in U.S. Pat. No. 5,825,482 uses oblique illumination and dark-field detection (i.e., the illumination approaches the wafer obliquely). A third type of system, as embodied in U.S. Pat. No. 5,982,921, uses normal illumination and dark field detection. All of these conventional approaches have advantages and disadvantages, some of which relate to the particular application or situation in which the system is used.
Under normal illumination, the surface of the object viewed is normal to the optic axis of the objective lens and light is used to illuminate the object. In a bright field system, light reflected back to the objective lens in a direction substantially parallel to the incident beam is used to form an image. Hence, surfaces that are reflective and perpendicular to the light rays appear bright and features that are nonreflective or oblique reflect less light back to the objective lens and appear darker. A dark field system may be implemented with either normal or oblique illumination. In either case, light that is scattered away from the optical axis is collected by dark field detectors positioned at an angle to the surface being viewed to form an image. Inclined surfaces of features such as ridges, pits, scratches, and particles therefore appear bright, providing enhanced contrast of these features from subtle topographic features. Thus, reflective features that normally appear bright in bright field illumination are completely black in darkfield illumination and subtle features that are undetectable using bright field illumination may be readily observed with dark field illumination.
In a laser-scan wafer inspection scenario it is sometimes preferable to illuminate the wafer at an angle normal to the wafer surface, while at other times preferable to use oblique illumination, depending on the details of the wafer materials, patterns and defects. The optical scattering characteristics of semiconductor wafers vary dramatically as the wafers proceed from one step to the next of the IC production flow. Some layers (such as bare silicon) are very smooth whereas some others (such as deposited aluminum) can be very rough and grainy.
It is well known that oblique illumination angles help reduce the unwanted optical scattering of the grains and roughness by the “Lloyd's mirror” effect (a destructive interference of the incident and reflected light at the surface which substantially reduced scatter from roughness and grains whose height from the surface is much less than the wavelength of the incident light, especially for metallic surfaces). Oblique illumination angles have, however, some limitations which make them less useful than normal illumination for some layers. One deficiency of oblique illumination angles is the inability of the light to penetrate between dense lines, such as those used in poly-silicon or metal interconnects. Another deficiency of oblique illumination is the dependence of the scattered signal on the direction of the substrate features (i.e., the loss of the symmetry which exists with normal illumination).
In practical inspection systems it is often desired to have replaceable optical elements which allow determination of the spot size. Such a system can thus be optimized for scanning with a large spot and obtaining a very high scan speed although a limited sensitivity; or, on the other hand, for scanning with a small spot and obtaining a very high sensitivity but at a lower scan speed. For normal illumination this is quite straightforward to do and only the classical resolution limits how small the spot can become. For oblique illumination, however, very small spots cannot be obtained due to the additional geometrical factor which introduces spot spread across the substrate plane which is inclined to it.
Accordingly, a need exists in the art for an improved wafer inspection system selectively and advantageously permitting use of either normal scanning illumination or oblique scanning illumination, based on the particular optical scattering characteristics of a semiconductor wafer at a time of inspection.